Materials for protection against electromagnetic interference (EMI) and/or lightning strike mitigation are useful in a number of applications, and are commonly used, for example, in airplanes and other aircraft. Traditionally, metals are used for protection against EMI AND/or for lightning strike mitigation because of their high conductivity, but are often undesirable because of their weight and machinability constraints. Over the last few years, this problem has been addressed through development of light weight conductive polymers, either by synthesizing intrinsically conducting polymers (ICP) or by the inclusion of conductive fillers (e.g., carbon black, carbon fibers, carbon nanotubes) in insulating matrices. However, the shielding effectiveness (SE) of these materials and lightning strike mitigation capability is not as high as that of metals, even with the inclusion of high volume fractions of conductive fillers in such polymers. Also, addition of high volume fractions of filler in a composite results in difficulty of manufacture and degradation in toughness and ductility.
One current composite approach uses 100 microns of copper wire in every graphite tow throughout the structure. This method suffers drawbacks due to the additional weight gain of 60 grams per square meter. This can add up to 500-1000 pounds to the overall aircraft. In addition, the conducting material evaporates after a single lightning strike, which leads to high repair costs.
In the case of conductive filler-filled insulating polymers, connectivity of filler particles is not as important for EMI shielding as for electrical conductivity (important for lightning strike mitigation), but the shielding effectiveness improves with a network formation because of enhanced conductivity increases the reflective capacity of the shield. Though carbon is an intrinsically conducting material (10-103 S/m), only high volume fractions of carbon black or short carbon fibers can make any insulating polymer matrix conductive enough to avoid accumulation of charge and to form a connecting and conducting network.
Composites have been manufactured containing SWNTs using a spraying technique. More specifically, in one example, a mixture of 1-3 wt % SWNT is suspended in a solution of dimethylformamide (DMF) or alcohol, and the mixture was sprayed onto the surface of a prepreg or fabric. The resulting composite provided a 42% increase in EMI shielding, but little change in surface conductivity. Additionally, difficulties were encountered in maintaining a desired amount of SWNTs on the prepreg or composite surface. Other composites have been manufactured using nickel-coated SWNTs with the same spraying technique. By spraying nickel-coated SWNTs (1.5 wt %) onto the surface of a prepreg, surface conductivities of 6-10 orders of magnitude higher than the base composite can be established. However, the use of nickel coated SWNTs also decreases the EMI shielding effectiveness (SE) of the material. It is therefore desirable to produce a composite having both enhanced surface conductivity and enhanced EMI shielding effectiveness.
Aspects of the present invention are provided to solve the problems discussed above and other problems, and to provide advantages and aspects not provided by prior materials of this type. A full discussion of the features and advantages of the present invention is deferred to the following detailed description, which proceeds with reference to the accompanying drawings.